A) Field of the Invention
The present invention relates to a solid state image pickup apparatus for color image pickup using a solid state image pickup device of a charge coupled device (CCD) type, a metal oxide semiconductor (MOS) type or another type as an area image sensor, and more particularly to a solid state image pickup apparatus using a simultaneous single-plate type color image pickup device.
B) Description of the Related Art
Solid state image pickup apparatuses for color image pickup using solid state image pickup devices as area image sensors are prevailing nowadays. Such solid state image pickup apparatuses are used, for example, in digital cameras, in digital cameras built in personal computers, personal digital assistants or the like.
Color shooting solid state image pickup apparatuses are roughly classified into a single-plate type and a three-plate type according to their image pickup methods. Single-plate type solid state image pickup apparatuses are further classified into a frame sequential type which time sequentially picks up color image information of each color, and a concurrent type which picks up color image information of all colors at the same time. A general purpose solid state image pickup apparatus generally uses a concurrent single plate type solid state image pickup device (hereinafter simply called a “solid state image pickup device”).
A solid state image pickup device has about several hundred thousand to several million color pixels. Each color pixel includes one photoelectric conversion element made of, e.g., a photodiode and one color filter disposed above the photoelectric conversion element (before the element in a light incidence course).
A solid state image pickup device has a predetermined number of color pixels including at least three color-types each of which detects color information different from each other. Color information detected by each color pixel is dependent upon a color of the color filter. For example, in the solid state image pickup device using primary color filters, the color pixels are classified into three color-types of red, green and blue. In the solid state image pickup device using complementary color filters, the color pixels are classified into three to four color-types. The complementary color filters often constitute one color filter array together with green color filters. Color pixels are disposed in a square matrix layout or in a pixel shift layout. The “square matrix layout” used in this specification is intended to include a matrix layout having different numbers of rows and columns.
The “pixel shift layout” used in this specification is intended to mean a number of color pixels disposed in the following manner. Each color pixel in an odd number column is shifted in the column direction from each pixel in an even number column by about a half of the pitch between color pixels in the column, whereas each color pixel in an odd number row is shifted in the row direction from each pixel in an even number row by about a half of the pitch between color pixels in the row, and each column contains only color pixels of either the odd rows or even rows. The “pixel shift layout” is one example of the layouts of a number of color pixels disposed in a plurality of rows and columns in a matrix shape.
The term “about a half of the pitch of color pixels in the column” is intended to include a precise half as well as a substantial half. Although the substantial half is not a precise half because of manufacture errors, rounding errors of pixel arrangement caused by design or mask manufacture, this value can be regarded as substantially the half from the viewpoint of the total performance of a solid state image pickup device and the image quality. The term “about a half of the pitch of color pixels in the row” is also intended to include a precise half as well as a substantial half.
As light becomes incident upon a color pixel, electric charge corresponding to the quantity of incidence light is accumulated in the photosensitive conversion element. A solid state image pickup device can sequentially output signals corresponding to the charges accumulated in color pixels. Each of these signals is called a “pixel signal” in this specification. Each pixel signal contains color information of substantially one color only.
A solid state image pickup apparatus generates a number of pixel signals for reproducing an image by using pixel signals sequentially output from a solid state image pickup device. In this specification, each of the pixel signals for reproducing an image is called an “output pixel signal”.
In accordance with output pixel signals from the solid state image pickup apparatus, an image constituted of a number of pixels is reproduced on a monitor or by a printer. In this specification, each of the pixels is called a “reproduction pixel”.
Spatial mixing is generally used for full-color printing by a printer and for full-color display by a monitor. Spatial mixing can be realized by additive mixing or subtractive mixing. In most cases, additive mixing of red, green and blue is performed. One pixel of a full-color image formed by additive mixing includes red, green and blue colors information. Each reproduction pixel includes therefore red, green and blue colors information.
A color shooting solid state image pickup apparatus generates three output pixel signals per one reproduction pixel, i.e., a red output pixel signal containing red color information, a green output pixel signal containing green color information, a blue output pixel signal containing blue color information. Three output pixel signals of red, green and blue per one reproduction pixel are called a “set of output pixel signals”.
If a solid state image pickup device uses complementary color filters, complementary color information is converted into primary color information, prior to executing an interpolation process to be later described.
As described earlier, a pixel signal of a color pixel of a solid state image pickup device contains color information of substantially one color only. Therefore, of color information to be contained in one set of output pixel signals, two pieces of color information not obtained from the pixel signal of one color pixel are generated through interpolation processes using signals based on pixel signals of other color pixels.
For example, of the color information to be contained in one set of output pixel signals for a reproduction pixel corresponding to one particular green pixel of the solid state image pickup device, green color information is obtained from a signal based on a pixel signal of one green pixel, red color information is generated by interpolation process using signals based on pixel signals of a plurality of red pixels disposed near the green pixel, and blue color information is generated by interpolation process using signals based on pixel signals of a plurality of blue pixels near the green pixel.
Most of solid state image pickup apparatuses have functions of generating output pixel signals at a relatively high resolution and at a relatively low resolution.
For example, output pixel signals generated at a relatively high resolution are stored in a recording medium and then printed out by a printer or the like to reproduce a fine still image. The number of output pixels is, for example, equal to the number of effective pixels of the solid state image pickup device, e.g., several hundred thousand to several million pixels.
If a solid state image pickup device having a number of color pixels disposed in the pixel shift layout is used, it is possible to generate output pixel signals, by interpolation, corresponding to virtual pixels each of which arranged between adjacent two color pixels in each color pixel column or each color pixel row of the solid state image pickup device. In this case, output pixel signals corresponding to about twice as many reproduction pixels as the total number of color pixels (effective pixels) of the solid state image pickup device can be obtained.
Output pixel signals generated at a relatively low resolution are output to a monitor of the solid state image pickup apparatus or another apparatus to reproduce a still image or moving image. The number of output pixels depends upon the number of pixels of the monitor.
The number of pixels of a monitor is determined by various standards. For example, 176×144 (about twenty five thousand) pixels according to the Quarter Common Intermediate Format (QCIF), 352×288 (about one hundred thousand) pixels according to the Common Intermediate Format (CIF), 640×480 (about three hundred thousand) according to the Video Graphics Array (VGA), and 1280×960 (about one million and two hundred thirty thousand pixels according to the Super Extended Video Graphics Array (SXVGA). Other standards are also known.
In order to reproduce a moving image having a smooth motion, it is necessary to output a number of output pixel signals in the predetermined order at a high frame frequency. A high process performance is therefore necessary for a signal processing circuit which generates output pixel signals. As a result, the productivity of a solid state image pickup apparatus becomes lower and the manufacture cost of the apparatus becomes higher.